Conversion of ac lines to hvdc lines

ABSTRACT

An electric power transmission system includes at each end of a high voltage direct current transmission line including three conductors, a converter station for conversion of an alternating voltage into a direct voltage for transmitting direct current between the stations in all three conductors. Each station has a voltage source converter and an extra phase leg connected between the two pole conductors of the direct voltage side of the converter. A third of the conductors is connected to a midpoint between current valves of the extra phase leg. An arrangement is adapted to control the current valves of the extra phase leg to switch for connecting the third conductor either to the first pole conductor or the second pole conductor for utilizing the third conductor for conducting current between the stations.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of transmission of electricpower through high voltage transmission lines, which may have conductorsin the form of overhead conductors or cables. The invention is notrestricted to any particular levels of such high voltages. Furthermore,it is pointed out that “conductor” is in this disclosure, in thedescription as well as in the claims, to be interpreted to coveroverhead lines as well as cables, such as for instance of PEX-type orany other conceivable type.

Electric power may be transmitted through AC power systems or through DCpower systems in the form of High Voltage Direct Current (HVDC)transmission systems.

AC power systems seldom reach the thermal maximum of the conductorsthereof, but transfer limits are more often set by characteristics ofthe electrical network of which a line is a part than by the thermallimitations of its conductors. Beyond a certain level of power transfersynchronism of the AC system may be jeopardized, voltages may becomedepressed or unstable or the inadvertent loss of the line in questioncould not be accommodated by other lines on the system.

High Voltage Direct Current (HVDC) transmission overcomes some of theselimitations of an AC system. Losses are reduced when transmittingelectric power in a DC system compared to an AC systems, especially whenthe voltage in question is high. Furthermore, HVDC lines may operate upto the thermal limits of the conductors thereof. Another advantage ofHVDC is that it is much more compatible with modern power brokering.Moreover, bipolar HVDC lines, equipped with metallic ground return canloose one conductor and still operate at half power.

It is for the above reasons understandable that the industrycontemplates conversion of selected existing AC lines to HVDC lines.

However, would an AC transmission system be converted to a HVDC systemand the AC system has a single circuit AC line with only threeconductors this means that two of the conductors will form the two polesof the HVDC transmission line, while the third conductor will only serveas an emergency ground should One normal pole be out of service. Thus,this would render the thermal limit of single circuit AC lines convertedto DC about the same as the prior AC limit. This problem is there forany other AC line with an odd number of conductors, such as nine.

PRIOR ART

U.S. Pat. No. 6,714,427 describes an electric power transmission systemand a method for operation thereof making it more attractive to convertAC systems into HVDC system, since it proposes to design the converterstations at each end of the transmission line for conversion of analternating voltage into a direct voltage for transmitting directcurrent between said stations in all three conductors. This is obtainedby providing each of the three conductors with a separate mono-polebridge of thyristor valves, which modulates the current so that shortperiods of over-current in the respective conductor are off-set by likeperiods of low current so as to obtain an acceptable rms current levelthereby taking advantage of the thermal time constant of the conductorsand equipment. This means that a DC current in one of the conductors mayover a period of time have a level above that conductor's nominalthermally-limited current-carrying capacity, whereas the current in theopposite direction between the two stations of the system are shared bythe other two conductors, and the current level in the conductors may bechanged for rotating the higher level current among the threeconductors. This means that the thermal limit of all three conductorsmay be utilized. When for instance the higher level is twice the lowerlevel the losses will be reduced by 25% with respect to the case of onlyusing two conductors. Thus, the method according to U.S. Pat. No.6,714,427 makes it really interesting to convert AC systems into DCsystems, since in the order of 50% more current may be transferred insaid line when all three conductors are used instead of only two.

However, a full twelve pulse converter with bi-directional valvesalternatively full anti-parallel valves are added to the converter in aconverter station according to U.S. Pat. No. 6,714,427 for being able touse all three conductors of the line with respect to only using twoconductors, which involves a considerable cost making the economicadvantage interesting only in cases where HVDC instead of AC is close tojustification anyway without using all three conductors.

The present invention relates to an electric power transmission systemcomprising at each end of a High Voltage Direct Current transmissionline comprising three conductors, a converter station for conversion ofan alternating voltage into a direct voltage for transmitting directcurrent between said stations in all three conductors as the systemdisclosed in U.S. Pat. No. 6,714,427.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an electric powertransmission system of this type, which is simplified with respect tothe known system discussed above and by that less costly than thatsystem making it even more attractive to convert existing AC systemswith transmission lines having three conductors into HVDC systems.

This object is according to the invention obtained by providing such anelectric power transmission system, which is characterized in that eachsaid converter station comprises a Voltage Source Converter (VSC) withone or more phase legs having current valves of semiconductor devices ofturn-off type and rectifying members in anti-parallel therewithconnected in series between a first and a second so-called poleconductor of said conductors and a midpoint being connected to analternating voltage side of the converter, and a control unit forswitching the current valves by controlling the semiconductor devicesthereof for converting said alternating voltage into a direct voltageapplied to said two pole conductors, that the station further comprisesan extra phase leg of current valves of semiconductor devices ofturn-off type and rectifying members connected in anti-paralleltherewith connected in series between said two pole conductors on thedirect voltage side of said converter, that a third of said conductorsis connected to a midpoint between current valves of said extra phaseleg, and that the station further comprises an arrangement adapted tocontrol said current valves of said extra phase leg to switch forconnecting said third conductor either to the first pole conductor orthe second pole conductor for utilizing the third conductor forconducting current between said stations.

Thus, by utilizing a VSC-converter in each converter station and onlyadding one extra phase leg all three conductors instead of only two maybe utilized for carrying current and by that transmitting power betweenthe stations. In the case of a three-phase alternating voltage appliedto the alternating voltage side of the converter this means that thereis only a need of ⅓ more current valves instead of twice as many currentvalves plus extra transformers as in the known system discussed above.This also means that the control to be carried out for the currentsharing of the three conductors, i.e. the control of the current valvesin said extra phase leg, will be simplified with respect to the controlof the valves in the known system involving reduced costs for thecontrol equipment and a higher reliability of the operation of thesystem.

According to an embodiment of the invention each said first and secondconductors are between said stations connected in series with a resistorand a circuit for by-passing the resistor, and the system comprisesmeans adapted to control said by-pass circuit for controlling currentsharing among the three conductors by conducting the current through therespective pole conductor through the resistor or by-passing theresistor. This means a possibility to efficiently obtain the currentsharing among the conductors aimed at.

According to another embodiment of the invention said means is adaptedto control the by-pass circuit to by-pass said resistor during periodsof time when the whole direct current in one direction between thestations is flowing in the respective pole conductor and to control thecurrent to flow continuously or at least a part of the time through theresistor during periods of time when the current flowing from onestation to the other is shared by the respective pole conductor and thethird conductor. The losses introduced by arranging said resistor are bythis eliminated during the period of time when there is no need forcurrent sharing, since the whole direct current is flowing in the poleconductor in question.

According to another embodiment of the invention said by-pass circuitcomprises a series connection of two oppositely directed current valvesof a semiconductor device of turn-off type and a rectifying memberconnected in anti-parallel therewith, and said means is adapted tocontrol the by-pass circuit by controlling said semiconductor devices.This is a suitable and simple way of realizing said by-pass circuit, inwhich for instance one or a few IGBT's may be used in such a currentvalve.

According to another embodiment of the invention the system comprisesmeans adapted to switch the current valve or current valves between theterminal of said third conductor and that of one of the two poleconductors in said extra phase leg of one of the stations at a frequencyfor carrying out a DC/DC-conversion for influencing the level of thepotential at said third conductor terminal connected to that phase legand by that the current flowing in said third conductor for regulatingcurrent sharing between this conductor and the one of the two poleconductors conducting current in the same direction as the thirdconductor between the stations. This means a possibility to obtainproper current sharing without resistor and by-pass circuit avoiding thelosses by the introduction of said resistor. The switching of thecurrent valve or current valves in question of said extra phase leg isthen created by PWM (Pulse Width Modulation), in which the switchingfrequency could be in the order of 1 kHz in order to avoid too bigfilters and substantial extra costs as a consequence of thisDC/DC-conversion switching.

According to another embodiment of the invention said arrangement isadapted to control the current valves of said extra phase legs of thestations so that a current of a substantially constant level is flowingin said third conductor in a direction changing while changing in whichone of the two pole conductors the full direct current in one directionbetween the stations is flowing. This constitutes an efficient andsimple way of controlling the current in the three conductors, and thearrangement is preferably adapted to carry out said control for makingsaid substantially constant level of the current in said third conductorbeing close to the thermal limits of the third conductor, so that thisthird conductor is fully utilized. This means that the two poleconductors are alternatingly carrying a current being above and belowthe rms thermal limit of that conductor. This is of course valid whenthere is a desire to transmit as much power as even possible through thetransmission system, and when the load is lower the current in thedifferent conductors will be lowered correspondingly, so that thecurrent in the third conductor will then also be substantially lowerthan said thermal limit of that conductor.

According to another embodiment of the invention said arrangement isadapted to control the current valves of said extra phase legs to makethe first and the third conductors share the direct current between thestations in one direction during a period of time followed by acorresponding period of time in which the second and the thirdconductors are sharing the current between the stations in the oppositedirection, and the arrangement is adapted to carry out the controlaccording to such periods of time being in the range of 20 seconds-30minutes, 30 seconds-10 minutes or 1-5 minutes. This means that theswitching of the extra phase leg in each station is for changing thedirection of the current in said third conductor and starting a newperiod of time only carried out with this frequency, which accordinglymay be less than once a minute. These periods of time may be longer forcables than for overhead lines.

According to another embodiment of the invention the system is designedto carry a voltage between said two pole conductors of 50 kV-1200 kV,especially above 100 kV, above 200 kV and above 400 kV. A HVDCtransmission system instead of an AC transmission system is of coursethe more interesting the higher said voltage is.

According to another embodiment of the invention said converter hasthree said phase legs for connecting a three-phase alternating voltageto the alternating voltage side thereof. Although this will normally bethe case, especially when an AC system having three conductors in atransmission line thereof is to be converted into a HVDC system, theinvention also covers the case of connecting a single-phase alternatingvoltage to the alternating voltage side of the converter.

The invention also relates to a converter station for converting analternating voltage into a direct voltage being a part of an electricpower transmission system according to the invention.

The invention also relates to a method for controlling the flow ofelectric power in an electric power transmission system comprising ateach end of a High Voltage Direct Current (HVDC) transmission linehaving three conductors, a converter station for conversion of analternating voltage into a direct voltage for transmitting directcurrent between said stations in all three conductors, according to theappended independent method claim. The advantageous features andadvantages thereof as well as of the methods according to theembodiments defined in the appended depending method claims appear fromthe discussion above of the corresponding claims for the systemaccording to the invention.

The invention also relates to a computer program as well as a computerreadable medium according to the corresponding appended claims. Thesteps of the method according to the invention are well suited to becontrolled by a processor provided with such a computer program.

Other advantages as well as advantageous features of the invention willappear from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a specificdescription of embodiments of the invention cited as examples.

In the drawings:

FIG. 1 is a schematic circuit diagram illustrating an electric powertransmission system according to a first embodiment of the invention,

FIG. 2 is a graph of the currents in the three conductors of thetransmission line in the system according to FIG. 1 versus timeaccording to a simulation model, and

FIG. 3 is a circuit diagram of an electric power transmission systemaccording to a second embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 schematically illustrates an electric power transmission systemaccording to a first embodiment of the invention, which has beensimplified for only showing the components necessary for explaining theinvention. This system comprises at each end of a High Voltage DirectCurrent (HVDC) transmission line 1 having three conductors 2, 3, 4, aconverter station 5, 6 for conversion of an alternating voltage into adirect voltage for transmitting direct current between said stations inall three conductors.

Each said converter station comprises a Voltage Source Converter (VSC)7, 8 with three phase legs 9-11 and 12-14, respectively, having currentvalves 15 of semiconductor devices 16 of turn-off type, such as IGBT's,and rectifying members 17, such as rectifying diodes, in anti-paralleltherewith connected in series between a first 2 and a second 4 so-calledpole conductor of said conductors and a midpoint 18 being connected toan alternating voltage side 19, 20 of the converter in the form of athree-phase alternating voltage network, generator, load or the like.

Although for the sake of simplicity only shown for one of the stations 5each converter station also comprises a control unit 21 for switchingthe current valves by controlling the semiconductor devices thereof forconverting said alternating voltage into direct voltage applied to saidtwo pole conductors 2, 4. The control unit will control the currentvalve according to a Pulse Width Modulation pattern by such switchingwith a frequency in the range of 1 kHz-10 kHz, through which the powerflow direction between the two stations may be controlled, i.e. whichone of the stations functions as rectifier and which one as inverter. Itis pointed out that a plurality of semiconductor devices and rectifyingmembers may be connected in series in each current valve for being ableto together hold the voltage to be held by the current valve in theblocking stage thereof.

The system described so far, except for said third conductor 3,corresponds to a conventional high voltage direct current bipolarsystem. To this is according to the invention in each station an extraphase leg 22, 22′ added, which has current valves 23-26 of semiconductordevices of turn-off type and rectifying members connected inanti-parallel therewith connected in series between the two poleconductors 2, 4 on the direct voltage side of the converter. The thirdconductor 3 is connected to a midpoint 27, 28 between current valves ofsaid extra phase leg.

Each said station further comprises an arrangement 29 adapted to controlsaid current valves of said extra phase leg to switch for connectingsaid third conductor either to the first pole conductor 2 or the secondpole conductor 4 for utilizing the third conductor for conductingcurrent between said stations.

The first 2 and second 4 conductors are between said stations connectedin series with a resistor 30 and a circuit 31 for by-passing theresistor. The by-pass circuit comprises a series connection of twooppositely directed current valves 32, 33 of a semiconductor device ofturn-off type, such as an IGBT, and a rectifying member, such as arectifying diode, connected in anti-parallel therewith. Means includedin said arrangement 29 are provided for controlling said by-pass circuitby controlling said semiconductor devices thereof.

The function of the transmission system according to FIG. 1 is asfollows. It is shown how the full current between the two stations iscarried by the second pole conductor 4, whereas the first pole conductor2 and the third conductor 3 are sharing the current flowing in the otherdirection. This means that the current valve 23 of the extra phase leg22 is switched to connect the midpoint 27 to the first conductor 2 forsharing the current therewith. The current through the first conductor 2will in this state be led through the resistor 30 for obtain propercurrent sharing, whereas the by-pass circuit in the second poleconductor 4 will be switched in for by-passing the resistor 30 belongingto that conductor carrying the full current. This may constitute thefirst period of time in the graph of FIG. 2, in which then the currentfor the second conductor II, the third conductor III and the firstconductor I are shown counted from above. The arrangement 29 may in thisstate control the current valves of the extra phase legs of the stationso that a current of a substantially constant level being close to thethermal limits of the third conductor is flowing in the third conductor.The arrangement is also adapted to control the extra phase legs to makethe first and the third conductors share the direct current between thestations in one direction during a period of time followed by acorresponding period of time in which the second and the thirdconductors are sharing the current between the stations in the oppositedirection as shown in FIG. 2. The current valves of said extra phaselegs may be switched in the order of once a minute for changing from onesuch time period to another, but in the simulations illustrated in FIG.2 such switching has been carried out once a second.

FIG. 3 illustrates a power transmission system according to a secondembodiment of the invention, which differs from the one according toFIG. 1 by the omission of the resistor with by-pass circuit in the twopole conductors for obtaining proper current sharing. Means 40 areinstead arranged for switching the current valve or current valvesbetween the terminal 27, 28 of said third conductor and that of one ofthe two pole conductors 2, 4 in a said extra leg 22, 22′ of one of thestations at a frequency for carrying out a DC/DC-conversion, such as 1kHz, for influencing the level of the potential at said third conductorterminal connecting to that phase leg and by that the current flow insaid third conductor for regulating current sharing between thisconductor and the one of the two pole conductors conducting current inthe same direction as the third conductor between the stations. Thiswould in the case of the current flow as shown in FIG. 3 means that thecurrent valve 15′ of the extra phase leg 22′ is switched at saidfrequency for carrying out said DC/DC-conversion for suitablyinfluencing the level of the potential in the point 28 for obtainingproper current sharing between the first conductor 2 and the thirdconductor 3. This embodiment gives possibility to modulate the currentswith great accuracy, and there is no need for extra valves of a by-passcircuit and big resistors to cool. However, there will be some switchinglosses as the extra phase leg in one of the stations will have to switchconstantly.

The invention is of course not in any way restricted to the embodimentsdescribed above, but many possibilities to modifications thereof will beapparent to a person with ordinary skill in the art without departingfrom the basic idea of the invention as defined in the appended claims.

The invention is applicable to any electric power transmission systemhaving a transmission line with at least three conductors, preferably anodd number of conductors, in which one or more groups of three of themare arranged according to the invention.

1. An electric power transmission system, comprising: a high voltagedirect current line transmission line comprising three conductors; and aconverter station arranged at each end of the high voltage directcurrent line transmission line, the converter station is configured toconvert an alternating voltage into a direct voltage for transmittingdirect current between said stations in all three conductors, eachconverter station comprises voltage source converter comprising at leastone phase legs having current valves of turn-off semiconductor devicesand rectifying members arranged anti-parallel the semiconductor devices,the rectifying members being connected in series between a first and asecond pole conductor of said conductors and a midpoint being connectedto an alternating voltage side of the converter, and a control unit forswitching the current valves by controlling the semiconductor devicesthereof for converting said alternating voltage into a direct voltageapplied to said two pole conductors, wherein the converter stationfurther comprises an extra phase leg of current valves of semiconductordevices of turn-off type and rectifying members connected inanti-parallel therewith connected in series between said two poleconductors on the direct voltage side of said converter, wherein a thirdof said conductors is connected to a midpoint between current valves ofsaid extra phase leg, and wherein the converter station furthercomprises an arrangement adapted to control said current valves of saidextra phase leg to switch for connecting said third conductor either tothe first pole conductor or the second pole conductor for utilizing thethird conductor for conducting current between said stations.
 2. Thesystem according to claim 1, wherein each said first and secondconductor is arranged between said stations connected in series with aresistor and a circuit for by-passing the resistor, the system furthercomprising: a control configured to control said by-pass circuit forcontrolling current sharing among the three conductors by conducting thecurrent through the respective pole conductor through the resistor orby-passing the resistor.
 3. The system according to claim 2, wherein thecontrol is adapted to control the by-pass circuit to by-pass saidresistor during periods of time when the whole direct current in onedirection between the stations is flowing in the respective poleconductor and to control the current to flow continuously or at least apart of the time through the resistor during periods of time when thecurrent flowing from one station to the other is shared by therespective pole conductor and the third conductor.
 4. The systemaccording to claim 2, wherein said by-pass circuit comprises a seriesconnection of two oppositely directed current valves of a turn-offsemiconductor device and a rectifying member connected in anti-paralleltherewith, and wherein the control is adapted to control the by-passcircuit by controlling said semiconductor devices.
 5. The systemaccording to claim 1, further comprising: a switch adapted to switch thecurrent valve or current valves between the terminal of said thirdconductor and the terminal of one of the two pole conductors in a saidextra phase leg of one of the stations at a frequency for carrying out aDC/DC-conversion for influencing the level of the potential at saidthird conductor terminal connecting to the phase leg and thereby thecurrent flowing in said third conductor for regulating current sharingbetween this conductor and the one of the two pole conductors conductingcurrent in the same direction as the third conductor between thestations.
 6. The system according to claim 1, wherein said arrangementis adapted to control the current valves of said extra phase legs of thestations so that a current of a substantially constant level is flowingin said third conductor in a direction changing while changing in whichone of the two pole conductors the full direct current in one directionbetween the stations is flowing.
 7. The system according to claim 6,wherein said arrangement is adapted to carry out said control for makingsaid substantially constant level of the current in said third conductorbeing close to the thermal limits of the third conductor.
 8. The systemaccording to claim 1, wherein said arrangement is adapted to control thecurrent valves of said extra phase legs to make the first and the thirdconductors share the direct current between the stations in onedirection during a period of time followed by a corresponding period oftime in which the second and the third conductors are sharing thecurrent between the stations in the opposite direction, and wherein thearrangement is adapted to carry out the control according to a period oftime being in the range of 20 seconds-30 minutes.
 9. The systemaccording to claim 1, wherein the system is designed to carry a voltagebetween said two pole conductors of 50 kV-1200 kV.
 10. The systemaccording to claim 1, wherein said converter has three said phase legsfor connecting a three-phase alternating voltage to the alternatingvoltage side thereof.
 11. A converter station for converting analternating voltage into a direct voltage, comprising: a voltage sourceconverter comprising at least one phase legs comprising current valvesof turn-off semiconductor devices and rectifying members arrangedanti-parallel with the semiconductor devices, the rectifying membersbeing connected in series between a first and a second pole conductor ofsaid conductors and a midpoint being connected to an alternating voltageside of the converter, and a control unit for switching the currentvalves by controlling the semiconductor devices thereof for convertingsaid alternating voltage into a direct voltage applied to said two poleconductors; an extra phase leg of current valves of turn-offsemiconductor devices and rectifying members connected in anti-parallelwith the turn-off semiconductor device connected in series between saidtwo pole conductors on the direct voltage side of said converter,wherein a third of said conductors is connected to a midpoint betweencurrent valves of said extra phase leg; and an arrangement adapted tocontrol said current valves of said extra phase leg to switch forconnecting said third conductor either to the first pole conductor orthe second pole conductor for utilizing the third conductor forconducting current between said stations.
 12. A method for controlling aflow of electric power in an electric power transmission systemcomprising at each end of a high voltage direct current transmissionline comprising three conductors, a converter station for conversion ofan alternating voltage into a direct voltage for transmitting directcurrent between said stations in all three conductors, the methodcomprising: carrying out the control for a system in which eachconverter station comprises a voltage source converter with one or morephase legs having current valves of turn-off semiconductor devices andrectifying members in anti-parallel therewith connected in seriesbetween a first and a second pole conductor of said conductors and amidpoint being connected to an alternating voltage side of theconverter, and a control unit for switching the current valves bycontrolling the semiconductor devices thereof for converting saidalternating voltage into a direct voltage applied to said two poleconductors, the station further comprising an extra phase leg of currentvalves of semiconductor devices of turn-off type and rectifying membersconnected in anti-parallel therewith connected in series between saidtwo pole conductors on the direct voltage side of said converter, inwhich a third of said conductors is connected to a midpoint betweencurrent valves of said extra phase leg, and controlling the currentvalves of said extra phase leg to switch for connecting said thirdconductor either to the first pole conductor or the second poleconductor for utilizing the third conductor for conducting currentbetween said stations.
 13. The method according to claim 12, wherein thecurrent through each of the pole conductors is controlled to either flowthrough a resistor or by-pass said resistor for controlling currentsharing among the three conductors.
 14. The method according to claim13, wherein when the whole direct current in one direction between thestations is flowing in a pole conductor this is controlled to by-passsaid resistor, and when the current flowing from one station to theother is by this pole conductor shared with the third conductor thecurrent is controlled to flow continuously for at least a part of thetime through the resistor.
 15. The method according to claim 12, whereinthe current valve or current valves between the terminal of said thirdconductor and that of one of the two pole conductors in a said extraphase leg of one of the stations is switched at a frequency for carryingout a DC/DC-conversion for influencing the level of the potential atsaid third conductor terminal connecting to that phase leg and by thatthe current flowing in said third conductor for regulating currentsharing between this conductor and the one of the two pole conductorsconducting current in the same direction as the third conductor betweensaid stations.
 16. The method according to claim 12, wherein the currentvalves of said extra phase legs of the stations are controlled so that acurrent of a substantially constant level is flowing in said thirdconductor in a direction changing while changing in which one of the twopole conductors the full direct current in one direction between thestations is flowing.
 17. The method according to claim 16, wherein saidcontrol of the current valves of said extra phase legs is carried out sothat said substantially constant level of the current in said thirdconductor is close to the thermal limits of the third conductor.
 18. Themethod according to claim 12, wherein the current valves of said extraphase legs are controlled to make the first and the third conductorsshare the direct current between the stations in one direction during aperiod of time followed by a corresponding period of time in which thesecond and the third conductors are sharing the current between thestations in the opposite direction, and that the control is carried outaccording to a period of time being in the range of 20 seconds-30minutes.
 19. A computer program product, comprising: a computer readablemedium; and computer program instructions recorded on the computerreadable medium and executable by a processor for carrying out a methodfor controlling a flow of electric power in an electric powertransmission system comprising at each end of a high voltage directcurrent transmission line comprising three conductors, a converterstation for conversion of an alternating voltage into a direct voltagefor transmitting direct current between said stations in all threeconductors, the method comprising: carrying out the control for a systemin which each converter station comprises a voltage source converterwith one or more phase legs having current valves of turn-offsemiconductor devices and rectifying members in anti-parallel therewithconnected in series between a first and a second pole conductor of saidconductors and a midpoint being connected to an alternating voltage sideof the converter, and a control unit for switching the current valves bycontrolling the semiconductor devices thereof for converting saidalternating voltage into a direct voltage applied to said two poleconductors, the station further comprising an extra phase leg of currentvalves of semiconductor devices of turn-off type and rectifying membersconnected in anti-parallel therewith connected in series between saidtwo pole conductors on the direct voltage side of said converter, inwhich a third of said conductors is connected to a midpoint betweencurrent valves of said extra phase leg, and controlling the currentvalves of said extra phase leg to switch for connecting said thirdconductor either to the first pole conductor or the second poleconductor for utilizing the third conductor for conducting currentbetween said stations.
 20. The computer program product according toclaim 19, wherein the computer program instructions are at leastpartially provided through a network.
 21. (canceled)